High tension fuse



July 11, 1933. T. E. MURRAY, JR

HIGH TENSION FUSE Filed Nov. 2l, 1928 4 NVENTOR A@fo/W45 MMR/4f L/f?.

BY HIS ATTORNEY 25 to slide freely in the case.

Patented July 11, 1933 UNITED STATES .PATENT OFFICE THOMAS MURRAY, JR., OF BROOKLYN, NEW YORK, ASSIGNOR TO METROPOLITAN DEVICE CORPORATION, A CORPORATION OF NEW YORK HIGH TENSION FUSE Application filed November 21, 1928. Serial No. 320,795.

In a previous application No. 297,527 filed August ll, 1928, now issued as U. S.' Patent No. 1,850,213 dated March 22, 1932, I have described a certain fuse and its combination 6 with the other parts of a circuit-breaking device.

The present application is directed to certain features of the fuse and its use either for protective purposes or in connection with A switching devices for positively Controlling electrical circuits.

Fig. 1 is a vertical longitudinal section of a fuse; Y

Fig. 2 is a central cross-section thereof;

Fig. 3 is a longitudinal sectional view of a fuse strip;

Fig. 4 is a central cross-section of the same.

The fuse is of a type covered in certain priorMurray applications, for example No.

Y 252,439 filed February 7, 1928 now issued as U. S. Patent No. 1,740,699 dated December 24, 1929.

An open ended tubular oase 1 carries a pair of plungers 2 of insulating material adapted Y The fusible element or strip 3 is embedded and fastened in the plungers and has a blowing peint 4 located between them. VThe gas generated by the blowing of the fuse is confined and the .C plungers with their engaged portions of the fuse strip are blown quickly away and out of the open ends of the casing to suoli a distance that the circuit cannot be re-'established by the passage of the current through the metallic gas generated.

l-hen the fuse blows the plungers are separated rapidly. But for an instant of time the electrical circuit is carried between the burnt ends ofthe fuse by the metallic vapor. 40 It is important to break this connection as quickly as possible, particularly where the fuse is used as a protection against short circuits or overloads. The intensity of a short circuit is proportioned to the length of time T taken to break it. As the time is lengthened,

the short is liable to build up enormous values. For example, a 100 ampere fuse on 240 volts might draw a load from the main line or generator of 100 times its normal capacity, that is, 10,000 amperes. Its efciency depends on the speedwith which it can clear the circuit.

With this type of fuse, I can effect a very' quick breaking of the circuit by making the fuse and plungers of the smallest proportions. The maximum speed is obtained by making the internal diameter of the tube as small as normal load conditions permit. The normal load conditions require certain standard cross-sections of the fuse strip at the blowing point, and the minimum size of my improved fuse is limited by this requirement. i v

Starting with a fuse strip which is standard for a given circuit, I make the inside di- Y ameter of the tube ofthe minimum size which will accommodate the fuse strip and plungers. The space between'the plungers should be as nearly as possible equal to the length of the reduced .sectionv of the strip. 'lhe thickness Vof the shell should be proportioned to the strength ofthe explosion and to the strength of course Vof the material of which it is constructed. Y The plungers should be as small and a vlight as possible.v For example, they may be of aluminum or an alloy of aluminum and brass. They must be long enough to travel smoothly and have mechanical strength and to retard the passage of gases around them.v

They fit'looselyl in the tube so as to oppose practically no frictional resistance; but their length is sufficient to prevent substantial loss of pressure withinthe brief time permitted. The leads,.or portions of the strip leading to the plungers are made as flexible as possible so as to offer the least resistance to the movement of the plungers.

The tube must be long enough to prevent the conduction of current around the outside of it through the vapors eXpelledat its ends. The length thereforeymust be increased proportionately to the voltage.`

In an actual case illustrated in Fig. 1, the fuse strip 3 is a standard 300 ampere fuse. It was placed in a circuit carrying 11,000 volts. The tube was one-half inch inside diameter and twelve inches long with a wall thickness of one-half an inch made of insulating fiber reinforced with metal 5. The plungers 2 were five-eighths of an inch long, spaced apart three-eighths of an inch. Such a fuse successfully opened a load of single phase cur rent of more than 240,000 kva at 11,300 volts; about 20,000 amperes in half a cycle. The current was sixty cycles and was therefore broken in one-one hundred and twentieth of a second.

As far as I am aware no fuse has ever been tried for breaking a circuit under such conditions. Elaborate and expensive oil switches controlled by relays have been used on such high tension lines to break the circuit in case of extraordinary overloads. But they would take a time interval of four or five cycles to break a circuit under the above conditions and, in such an extreme case, it is doubtful if the oil switch lwould open without serious damage.

In another case, I have used a tube 21/2 inches long and 1%; of an inch in diameter with a combustion chamber the space between the plungers) of .027 to .497 lcubic inches and such'a fuse has operated satisfactorily under short circuit condlitions on 2500l volts, 60 cycles, using 100 amperes. It opened the circuit in less than 1/4 of a cycle. This represents a time of less than 1/240 of a second, a speed previously unheard of in electrical practice.

Roughly speaking, the length of the tube should be at least about eight times its internal diameter. For example, a 1/2 inch tube for 2600 volts should be four to five inches long. For 6600 volts, six vto eight inches and for 11,000 volts, twelve to fourteen inches. If the inside diameter is changed, the length should be changed in proportion. If the above fuses have a diameter of one inch, their lengths should be double. These figures are approximate. But they represent fairly the best proportions available with the ordinary yfuse ribbons to secure the quickest complete clearing of a circuit.

Other metals than the ordinary fuse metal, which is usually a composition of zinc, would make it possible to reduce still further the size of the fuse and the quickness of its operation. Silver, for example, having a higher electrical conductivity, would permit the use of small cross-section compared with the ordinary fuse metal for the same high amperage capacity.

Additional means may be used to accelerate the rate of expansion, such as the introduc- In Figs. 1 and 2, for example, small pieces of gun cotton 6 are shown attached to the face of one of the plungers. One of the 'plungers may be fixed constituting in fact an abutment particularly in the smaller sizes of fuse. For the larger sizes, two movable plungers are preferred. This avoids any back kick or reaction on the casing so that it may be fixed as illustrated at its central portion in a standard or panel 7. The construction of the open ends is more economical than if one end were closed.

It is preferable that the plungers be fixed tightly on the strip; but they might be loose thereon if provision be made for them to engage the parts of the strip so as to carry such parts quickly out of the tube with the plungers. The chamber can be reduced or enlarged in diameter between the plungers accord-y ing to the time allowed for the explosive effect. The simplest construction, however, is to have the bore of uniform diameter throughout its length and to have the plungers tightly iixed on the strip so as to constitute in operation a part thereof.

Fig. 3 shows a Vstrip 3 of circular cross section with slight enlargements 8 at each side of the blowing point 4 and forming integral plungers. Such a fuse may be used with the same casing as in Fig. 1 Vand' with the additional pieces of gun cotton. But it would probably be of value chiefiy in the smaller sizes where gun cotton and a vrein-` at least one of said plunger members being 'freely movable within the said casing and adapted to be expelled therefrom as a result of the evolution of gases within the said casing upon the blowing out of said fuse wire; at least the inner part of said casing being of insulating character and said casing being adapted to resist disruption upon a blowing out of said fuse wire, the inside diameter of said casing approximating but being greater than the diameter of said fuse wire,

and the length of said casing being proportioned to the anticipated voltage to be applied across the said fuse, and ranging upward from about eight times the said insidediameter of the casing.

2. In an electrical fuse comprising an elongated tubular casing, a fuse wire having a blowout point disposed within said casing, plungermembers disposed within said casing and attached to said fuse wire at points adjacent to either side of said blowout point, at least one of said plunger members being `freely movable within the Vsaid casing and adapted to be expelled therefrom as a result of the evolution of gases within the saidcasing upon the blowing out of said fuse wire and a proportion of explosive material disposed within the said casing adjacent the said blowout point to facilitate the expulsion of said plunger upon the blowing out of said fuse wire; a casing comprised of insulating material reinforced with metal to adapt the said casing to resist disruption upon a blowing out of said fuse wire, the inside diameter of said casing approximating but being greater than the diameter of said fuse wire, and the length of said casing being proportioned to the anticipated voltage to be applied across the said fuse, and ranging upward from about eight times the said inside diameter of the casing.

3. In an electrical fuse comprising an elongated tubular casing, a fuse wire having a blowout point disposed within said casing, plunger members disposed within said casing and attached to said fuse wire at points adjacent to either side of said blowout point, at least one of said plunger members being freely movable within the said casing and adapted to be expelled therefrom as a result of the evolution of gases within the said casing upon the blowing outy of said .fuse wire; a casing comprised of insulating material reinforoed with metal to adapt the said casing to resist disruption upon a blowing out of said fuse wire, the insidediameter of said casing approximating but being greater than the diameter of said fuse wire, and the length of said casing being proportioned to the anticipated voltage to be applied across the said fuse, and ranging upward from about eight times the said inside diameter of the casing and the said plunger members being comprised of relatively light metal, the diameter of said plungers approximating but being less than the said inside diameter of said casing and having a length adapted to prevent substantial loss of pressure therearound during the expulsion of said plunger from the casing upon the blowing out of said fuse.

In witness whereof, I have hereunto signed my name.

THOMAS E. MURRAY, JR. 

